1. Field
The present invention relates to the treads of tires and to the rubber compositions used for the manufacture of such treads.
It relates more particularly to the treads of tires reinforced with reinforcing inorganic fillers such as siliceous or aluminous fillers, these treads being intended in particular for tires that are fitted to motor vehicles.
2. Description of Related Art
Ideally, a tire tread must meet a large number of often conflicting technical requirements, including a high wear resistance, a low rolling resistance, a high grip both on dry ground and on wet, snowy or icy ground, while giving the tire a very good level of handling on a motor vehicle, in particular a high drift thrust or cornering.
To improve the handling it is known that a greater stiffness of the tread is desirable, it being possible for this stiffness of the tread to be obtained for example by increasing the amount of reinforcing filler or by incorporating certain reinforcing resins into the constituent rubber compositions of these treads.
However, such a stiffness of the tread, at the very least for its surface portion that is in contact with the ground when the tire is running, as is known adversely affects, usually prohibitively, the grip properties.
In order to satisfy these two contradictory requirements, which are the handling and the grip, one solution consists in creating a stiffness gradient via an accommodation phenomenon of the rubber composition of the tread as is described in patent application WO 02/10269. This accommodation phenomenon is expressed by the ability of the rubber composition to become less stiff at the surface of the tread under the effect of the strains undergone by the tread when the tire is running. This reduction in the stiffness at the surface of the tread does not occur, or occurs only slightly, on the inside of the tread, which thus retains a higher level of stiffness than the surface of the tread. This stiffness gradient of the rubber composition has the advantage of being long-lasting, since the accommodation phenomenon occurs throughout the lifetime of the tire when it is running.
In view of the aforegoing, it is a constant objective to provide rubber compositions for tire treads that satisfy a compromise of performance between grip and handling.
This objective is achieved in that the inventors have discovered that the use of certain thermoplastic polymers as a non-reinforcing filler in a rubber composition reinforced by an inorganic filler gave the rubber composition, unexpectedly, a stiffness gradient and thus made it possible to improve the compromise of performances which are the handling and the grip, especially on wet ground, of a tire having a tread that is based on such a composition.
The use of thermoplastic polymers is well known in rubber compositions, as demonstrated by the publication of numerous documents relating to this subject, such as for example patent applications EP 0 867 471 A1, EP 1 008 464 A1 or US 2002/0019484. More particularly, polyvinylaromatic thermoplastic polymers having a weight-average molar mass between 2000 g/mol and 50000 g/mol are known for being used in rubber compositions for tread in order to increase the hysteresis of the rubber compositions comprising them without adversely affecting their breaking property. Such rubber compositions are described in patent applications EP 1 748 052 A1 and JP 2006-274045, which actually show an increase in the hysteresis without degrading the breaking strength.